Hydrogen is one of the fuels which will most likely replace fossil fuels in the future. Therefore a lot of manpower and material resources have been invested in research and development of hydrogen fuel in recent years. Now, the focus of such development is on hydrogen production, hydrogen storage, hydrogen transport and the combination of hydrogen and fuel cells. In early days, United States Department of Energy (U.S.DOE) had proposed a hydrogen storage standard for a hydrogen powered vehicle based on achieving a driving range of 500 km. The standard hydrogen storage is aimed to be 9 wt % by 2015.
Hydrogen has a major by-product, water, which has several advantages such as low pollution. Further, because hydrogen is a secondary fuel and it is abundant and recyclable, a more efficient and diverse supply and demand system of sustained energies may be constructed as long as technical difficulties in production, storage, transport and application are overcome. Thus every country considers it as the most preferable secondary fuel to replace fossil fuel, so as to increase national energy security and reduce air pollution. Therefore, development and application of hydrogen have received a great deal of attention recently. Hydrogen storage technology has to be enhanced in order to easily use hydrogen. The goals of hydrogen storage technology includes: gravimetric storage capacity, volumetric storage capacity, hydrogen release rate, and operating temperature etc.
The present method of hydrogen storage is mainly high-pressure storage or low-temperature storage (20K) system in which liquid form is most preferred. The current gravimetric storage density is about 15 wt % and the highest density reaches 18 wt %. However, a gravimetric storage density of only about 2.3 kW/hl is not ideal. There is still a gap between the current storage technology and the goal set for 2015. Furthermore, the temperature of hydrogen storage is too low to meet the demand for real application in reality. Thus the present study of hydrogen fuel is focused on a high storage capacity at normal temperature.
Research of physical absorption and chemical absorption between hydrogen and materials, a major factor of hydrogen storage, is mainly focused on active carbon, single-wall carbon nanotubes, and microporous metal-organic frameworks (MOFs), which is the most popular research topic now. A sample pellet of hydrogen storage prepared by said three materials will absorb a large amount of hydrogen by physical absorption and chemical absorption.
From a considerable amount of previous research, a high hydrogen storage capacity at normal temperature is believed to be an important index for application in reality and play a key role in carbon materials in the future to achieve the goal of effective hydrogen storage. In addition, in order to improve physical absorptive ability, there must be a breakthrough in the following three factors affecting hydrogen storage ability including: binding energy of hydrogen to absorption materials, surface area provided for hydrogen storage, and bulk density of sample pellet of hydrogen storage.
As for hydrogen storage of carbon materials, the use of active carbon is the main developing point in which the development of Pt/AC materials is improved increasingly, wherein the gravimetric storage capacity thereof is greater than 11 wt % in 6.9 MPa at room temperature. However, the apparent density is only about 0.35 g/cm3 after supported by Pt particles in proper proportion by weight due to the distribution of active carbon powder with size of 0.1˜20 μm and specific surface area of about 1800 m2/g. Thus, the converted volumetric hydrogen storage density is only about 38 KgH32/m3. Such a hydrogen storage capacity filled in a hydrogen cartridge is difficult to satisfy the DOE's target.
AC pellets were formed in a previous method by directly pressing AC powders at room temperature, and the apparent density will be substantially increased to about 0.9 g/cm3. However, the catalyst Pt particle did not uniformly distribute throughout AC pellets even with vacuum suction method, causing the substantial decrease in gravimetric hydrogen absorbing capacity. Therefore, how to prepare a Pt/AC pellet with slightly increased apparent density as well as increased volumetric hydrogen storage capacity is still a challenge.